Method and circuit for controlling baseband gain

ABSTRACT

An object of the present invention is to suppress the generation of a transient voltage in the settings of gains of a plurality of variable gain amplifiers of a baseband circuit. A gain converting circuit provides a limiting value to the quantity of the change of a gain which can be changed once. If gain input data has large change and the gain change exceeding the limiting value is carried out, the quantity of the change is divided into a plurality of quantities of change equal to or lower than the limiting value and variable gain amplifiers are controlled to thereby realize a required gain change. A gain distribution circuit distributes gain control data to the respective variable gain amplifiers on the basis of gain output data of the gain converting circuit.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field of the Invention

[0002] The present invention relates to a baseband gain control andparticularly to a baseband gain control method and circuit capable ofeffectively preventing problems derived from a DC offset in the gaincontrol of a direct conversion baseband circuit or the like.

[0003] 2. Description of the Prior Art

[0004] A receiver utilizing direct conversion is advantageous overconventional super-heterodyne type receiver in the following respectsand, therefore, expected to be widely used in the future:

[0005] 1) A high frequency circuit section is simplified and the numberof parts such as a filter can be reduced.

[0006] 2) Since most of the functions including band limitation and AGC(automatic gain control) are executed at a baseband frequency, they canbe realized by a CMOS analog circuit suited for LSI.

[0007]FIG. 6 is a view showing the concrete constitution of a directconversion receiver. FIG. 6 shows a baseband gain control system forcontrolling the gain of a direct conversion baseband circuit, e.g., asystem which has a wide dynamic range in the reception signals of areceiver of such a type as W-CDMA (Wide Band Code Division MultipleAccess).

[0008] A high frequency signal received by an antenna 201 is subjectedto band-limitation by a high frequency band-pass filter 202 and areceived band is taken out. The signal thus band-limited is amplified bya low noise amplifier LNA 203 and directly inputted into a quadraturedemodulator 204. The quadrature demodulator 204 is driven by a localsignal generated by a local oscillator 225. The frequency of this localsignal is the same as the central frequency of the received highfrequency signal.

[0009] The quadrature demodulator 204 consists of multiplicationcircuits 222 and 223 and a phase circuit 224. The balanced outputs ofthe low noise amplifier LNA 203 are multiplied by the multiplicationcircuits 222 and 223 through an amplifier 221 in response to thebalanced outputs of an orthogonal signal having a phase of 0° and thatof 90° of the local signal, respectively, a baseband signal is directlygenerated from the high frequency signal, and two types of signals,i.e., baseband signals I and Q, are outputted as demodulated outputs.These baseband signals I and Q are subjected to band-limitation bybaseband filters 205 and 206, respectively, and then amplified by an AGCcircuit 207 so as to have a constant average amplitude.

[0010] The dynamic range of the AGC circuit 207 has characteristics ofreaching several tens of decibels (about 80 dB for CDMA). The outputs ofthe AGC circuit 207 are outputted to the next stage as signals 215 and216, respectively. It is noted that a circuit controlling the gain ofthis circuit and the algorithm thereof are unrelated to the presentinvention and, therefore, not described herein.

[0011] According to the direct conversion system, channel filters forsuppressing adjacent channels are realized not by SAW filters for an IFband but by the baseband filters 205 and 206. Since they can be realizedby circuits using active elements, the baseband filters 205 and 206 aresuited for an IC. In addition, since the high frequency signal isdirectly converted into the baseband signals, there is no need toprovide a second local oscillator. For these reasons, there is aprobability that all the reception circuits from the low noise amplifierLNA 203 to the baseband outputs can be realized by one chip. Thisgreatly contributes to making a cellular phone smaller in size and tothe reduction of the number of parts.

[0012] Nevertheless, if there is a DC offset, even slightly, in thebaseband filters 205 and 206 and the AGC circuit 207, the gain of theAGC sometimes becomes as high as 80 dB and a saturation phenomenonoccurs that outputs are fixed to a power supply or the ground. Forexample, if there exists a DC offset of 1 mV in the bandpass filter 205and the gain of the AGC circuit 207 is 80 dB, i.e., 10,000 times as highas an input, a DC component of 10 V is outputted. Needless to say, sucha voltage is far beyond the voltage of a battery for a cellular phone,with the result that the cellular phone cannot operate.

[0013] As stated above, it is the most significant problem with thebaseband circuit of the direct conversion circuit to eliminate a DCoffset as much as possible.

[0014] There have been conventionally used high-pass filters (C-cut)each consisting of a DC cut capacitor or the like and provided betweenstages of variable gain amplifiers so as to eliminate the DC offset of abaseband circuit.

[0015]FIG. 7 is a view showing that the baseband circuit for I or Qshown in FIG. 6 is taken out. The baseband circuit consists of aplurality of gain control amplifiers having C-cut structures. Tosimplify description, FIG. 7 shows the baseband circuit as a single-endcircuit. A baseband filter 101 and variable gain amplifiers 102, 103 and104 (which amplifiers may be also referred to as “VGA1”, “VGA2” and“VGA3”, respectively) correspond to the baseband filter 205 (206) andthe variable gain amplifiers 208 (211), 209 (212) and 210 (213),respectively.

[0016] According to this structure, for the purpose of preventing thepropagation of a DC offset and the saturation of a signal due to thepropagation thereof, high-pass filters 109 to 111 corresponding toC-cuts are inserted between the input section of the circuit and the VGA102, the VGA 102 and VGA 103, the VGA 103 and the VGA 104 and the VGA104 and the output section, respectively. The gains of the VGA 1, VGA2and VGA 3 are controlled by gain control data distributed from the gaindistribution circuit 112 based on gain data inputted from externally.

[0017] As stated above, by inserting the high-pass filters into thebaseband circuit in appropriate units of the circuit, the propagation ofa direct current is prevented in a static state in which gains have nochange. In addition, the saturation of a signal due to the DC offset canbe prevented.

[0018] However, according to the conventional method for eliminating aDC offset in the baseband circuit of the direct conversion receiver, atransient phenomenon due to the DC offset occurs in a dynamic controlstate in which gains have great change, which often has an adverseeffect on reception characteristics.

[0019] Assuming that offset voltages V_(of1), V_(of2) and V_(of3) areadded to the input sides of the VGA1, VGA2 and VGA3, respectively, basedon the circuit of FIG. 7, it is considered what type of a transientphenomenon occurs to an output if the respective gains g₁, g₂ and g₃ arechanged.

[0020] It is assumed here that the transfer functions of the high-passfilters 109 to 111 inserted as shown in FIG. 7 are the same andrepresented by the following expression for brevity. $\begin{matrix}{{B(s)} = {\frac{s}{s + \alpha}.}} & (1)\end{matrix}$

[0021] It is assumed that the gains of the VGA1, VGA2 and VGA3 (not asdB values but as true values) are g₁, g₂ and g₃, respectively, and thatthese gains are changed to g₁′, g₂′ and g₃′, respectively. For brevity,the following conditions are set:

[0022] a) The gains g₁, g₂ and g₃ are 1 time to 16 times as high asinputs;

[0023] b) The gains g₁, g₂ and g₃ are not changed simultaneously; and

[0024] c) The gains g₁, g₂ and g₃ are changed instantaneously.

[0025] 1) If the gain of the VGA3 is changed from g₃ to g₃′:

[0026] Since being cut by the high-pass filters 109 and 110,respectively, the offset voltages V_(of1) and V_(of2) have no effect onthe output and only the offset voltage V_(of3) has an effect on theoutput. At the input of the high-pass filter 111, a step-like voltagechange ΔV₃ occurs as follows.

ΔV ₃=(g ₃ ′−g ₃)·v _(of3)  (2).

[0027] This step-like change influences an output V_(out) through thehigh-pass filter 111. A contribution thereof is described using Laplacetransform as follows. $\begin{matrix}{V_{{out}{(s)}} = {{{B(s)} \cdot \frac{\Delta \quad V_{3}}{s}} = {\left( {g_{3}^{\prime} - g_{3}} \right) \cdot V_{of3} \cdot {\frac{1}{s + \alpha}.}}}} & (3)\end{matrix}$

[0028] Assuming that g₃ is changed at t=0, a time response is obtainedas follows.

V _(out)(t)=(g ₃ ′−g ₃)·V _(of3) ·e ^(−αt)  (4).

[0029] 2) If the gain of the VGA2 is changed from g₂ to g₂′:

[0030] Because of the high-pass filter 110, the offset of the output ofthe VGA2 is cut by the filter 110 in a steady state. Then, it is assumedthat g₂ is changed to g₂′. At this moment, the following step-likevoltage change ΔV₂ occurs to the input of the high-pass filter 110.

ΔV ₂=(g ₂ ′−g ₂)·V _(of2)  (5).

[0031] This step-like change influences the output V_(out) through twostages of the high-pass filters. A contribution thereof is describedusing Laplace transform as follows. $\begin{matrix}{V_{{out}{(s)}} = {{g_{3} \cdot {B(s)}^{2} \cdot \frac{\Delta \quad V_{2}}{s}} = {{g_{3} \cdot \Delta}\quad {V_{2} \cdot \frac{s}{s + \alpha} \cdot {\frac{1}{s + \alpha}.}}}}} & (6)\end{matrix}$

[0032] Assuming that g₂ is changed at t=0, a time response is obtainedas follows:

V _(out)(t)=g ₃ ·ΔV ₂·(1−α·t)·e ^(−αt) =g ₃·(g ₂ ′−g ₂)·V_(of2)·(1−α·t)·e ^(−αt)  (7).

[0033] 3) If the gain of the VGA1 is changed from g₁ to g₁′:

[0034] Because of the high-pass filter 109, the offset of the output ofthe VGA1 is blocked by the filter 109 in a steady state. Then, it isassumed that g₁ is changed to g₁′. At this moment, the followingstep-like voltage change ΔV₁ occurs to the input of the high-pass filter109.

ΔV ₁=(g ₁ ′−g ₁)·V _(of1)  (8).

[0035] This step-like change influences the output V_(out) through threestages of the high-pass filters. A contribution thereof is describedusing Laplace transform as follows. $\begin{matrix}{V_{{out}{(s)}} = {{g_{3\quad} \cdot g_{2} \cdot {B(s)}^{3} \cdot \frac{\Delta \quad V_{1}}{s}} = {{g_{3} \cdot g_{2} \cdot \Delta}\quad {V_{1} \cdot \frac{s}{s + \alpha} \cdot \frac{s}{s + \alpha} \cdot {\frac{1}{s + \alpha}.}}}}} & (9)\end{matrix}$

[0036] Assuming that g₁ is changed at t=0, a time response is obtainedas follows. $\begin{matrix}{\begin{matrix}{V_{{out}{(t)}} = {{g_{3} \cdot g_{2} \cdot \Delta}\quad {V_{1} \cdot \left( {1 - {2 \cdot \alpha \cdot t} + \frac{\alpha^{2} \cdot t^{2}}{2}} \right) \cdot ^{{- \alpha}\quad t}}}} \\{= {g_{3} \cdot g_{2} \cdot \left( {g_{1}^{\prime} - g_{1}} \right) \cdot V_{of1} \cdot \left( {1 - {2 \cdot \alpha \cdot t} + \frac{\alpha^{2} \cdot t^{2}}{2}} \right) \cdot ^{{- \alpha}\quad t}}}\end{matrix}.} & (10)\end{matrix}$

[0037]FIG. 8 shows the waveform of the mathematical expression (4) ifthe offset voltage V_(of3) is 1 mV and the gain g₃ is changed from 1time to 16 times as high as the input.

[0038]FIG. 9 shows the waveform of the mathematical expression (7) ifthe offset voltage V_(of2) is 1 mV and the gain g₃ is 16 times as highas the input and the gain g₂ is changed from 1 time to 16 times as highas the input.

[0039]FIG. 10 shows the waveform of the mathematical expression (10) ifthe offset voltage V_(of1) is 1 mV, the gains g₃ and g₂ are 16 times ashigh as the inputs and the gain g₁ is changed from 1 time to 16 times ashigh as the input.

[0040] In any case, the 3 dB-cutoff frequency of each high-pass filteris 5 kHz and the value of α is 31415.93.

[0041] As is obvious from FIGS. 8 to 10, even if a direct currentcomponent can be blocked by the high-pass filters, a high transientvoltage occurs to the outputs and deteriorates characteristics bychanging the gain of each stage inadvertently.

[0042]FIG. 8, for example, shows that if the gain g₃ of the VGA3 ischanged from 1 time (0 dB) to 16 times (24 dB) as high as the input withthe DC offset voltage V_(of3) of 1 mV, a transient voltage pulse of 1mV×(16−1)=15 mV occurs. FIG. 9 shows that if the gain g₃ is 16 times ashigh as the input (24 dB) and the gain of the VGA2 is changed from 1time (0 dB) to 16 times (24 dB) as high as the input with the DC offsetvoltage V_(of2) of 1 mV, a transient voltage pulse of 1 mV×16×(16−1)=240mV occurs.

[0043] Further, FIG. 10 shows that if the gain g₃ of the VGA3 is 16times as high as the input (24 dB), the gain g₂ of the VGA2 is 16 timesas high as the input (24 dB) and the gain g₁ of the VGA1 is changed from1 time (0 dB) to 16 times (24 dB) as high as the input, a transientvoltage pulse of 1 mV×16×16×(16−1)=3840 mV occurs.

[0044] It is understood, therefore, that if the gains of a plurality ofvariable gain amplifiers are changed at random, a high transient voltageoccurs to the output even with a low offset voltage. This transientvoltage greatly damages the characteristics of the receiver.

[0045] As stated above, according to the direct conversion typereceiver, it is necessary to control gains almost in a basebandfrequency. Therefore, the saturation of amplifiers disadvantageouslyoccurs due to the DC offset which occurs to the respective sections ofthe baseband circuit. To prevent this, a method for blocking thepropagation of a DC component by providing high-pass filters atappropriate places of a circuit may be considered. In this case,however, a transient voltage occurs and deteriorates receptioncharacteristics depending on gain change.

SUMMARY OF THE INVENTION

[0046] An object of the present invention is to provide a baseband gaincontrol method and circuit capable of suppressing the generation of atransient voltage in the setting of the gains of a plurality of variablegain amplifiers in a baseband circuit.

[0047] The present invention is directed to control a baseband gain bysetting gains of a plurality of variable gain amplifiers amplifying abaseband signal and connected to one another in series, to decrease thegeneration of a voltage due to a transient phenomenon. The settings ofthe gains of the plurality of variable gain amplifiers in the basebandcircuit are controlled as follows:

[0048] 1. A limitation is set to the quantity of the change of a gainwhich can be changed at one time. If a gain change exceeding thelimiting value is necessary, the gain change is divided into a pluralityof quantities of change each equal to or lower than the limiting valueand a required gain change is attained while controlling the pluralityof quantities of change a plurality of times.

[0049] 2. If the gain is to be increased, the gains of the variable gainamplifiers starting at the variable gain amplifier close to an input aresequentially increased. If the gain is to be decreased, the gains of thevariable gain amplifiers starting at the variable gain amplifierfarthest to the input are sequentially decreased.

[0050] 3. The generation of a transient voltage is suppressed by thegain control by the method of 1 or 2 or the combination of the methods 1and 2.

[0051] According to the present invention, gain control is carried outso that the upper limit of the quantity of the change of the gains of aplurality of variable gain amplifiers for a baseband signal is set.Further, if the gain is to be increased, the gains of the variable gainamplifiers starting at the variable gain amplifier close to an input aresequentially increased. Further, if the gain is to be decreased, thegains of the variable gain amplifiers starting at the variable gainamplifier farthest to the input are sequentially decreased. Thus, it ispossible to effectively suppress the generation of a transient voltagecaused by a DC offset during the gain control.

[0052] The combination of the gain control for providing the upper limitof the quantity of the change of a gain and the distribution control fordistributing different gains to a plurality of variable gain amplifiersaccording to the increase and decrease of the gain enables, inparticular, suppressing the generation of a transient voltage caused bya DC offset more effectively.

[0053] If the present invention is applied to the gain control of adirect conversion baseband circuit, e.g., the baseband gain control of adirect conversion baseband circuit having a wide dynamic range of areception signal such as a receiver of a W-CDMA (Wide Band Code DivisionMultiple Access) type, the present invention exhibits considerably greatadvantage.

BRIEF EXPLANATION OF THE DRAWINGS

[0054]FIG. 1 is a view showing one mode for carrying out a baseband gaincontrol circuit of the present invention.

[0055]FIG. 2 is a flow chart showing the operation of a gain convertingcircuit in this mode for carrying out the invention.

[0056]FIG. 3 shows a manner in which the gain of a variable gainamplifier follows gain control data.

[0057]FIG. 4 is a flow chart showing the operation of a gaindistribution circuit.

[0058]FIG. 5 is a view showing an example in which the gain distributioncircuit is constituted out of an ROM.

[0059]FIG. 6 is a view showing the constitution of a conventional directconversion receiver.

[0060]FIG. 7 is a view showing the baseband circuit of the circuit shownin FIG. 6 as a single-end circuit for brevity.

[0061]FIG. 8 shows the waveform of an expression (4) if V_(of3) is 1 mVand g₃ is changed from 1 time to 16 times as high as an input.

[0062]FIG. 9 shows the waveform of an expression (7) if V_(of2) is 1 mVand g₃ is 16 times as high as the input and g₂ is changed from 1 time to16 times as high as the input.

[0063]FIG. 10 shows the waveform of an expression (10) if V_(of1) is 1mV and g₃ and g₂ are 16 times as high as the input and g₁ is changedfrom 1 time to 16 times as high as the input.

PREFERRED EMBODIMENT OF THE INVENTION

[0064]FIG. 1 is a block diagram showing the basic constitution of abaseband gain control circuit of the present invention. A signal pathshown therein has, similarly to the conventional circuit shown in FIG.7, a constitution of the reception signal baseband gain control systemof a receiver such as a W-CDMA (Wide Band Code Division Multiple Access)system. The block diagram of FIG. 1 shows a single-end circuit as in thecase of the description of the conventional circuit.

[0065] (First Mode of the Invention)

[0066] The first mode for carrying out the present invention ischaracterized by comprising a gain distribution circuit 112 and a gainconverting circuit 113 as shown in FIG. 1 and in that an upper limit ofthe gain control is set to the gain control circuit 113.

[0067] The gain converting circuit 113 is a circuit converting inputtedgain data (Gain Data: corresponding to dB) into gain output data (GainOutput: corresponding to dB) actually set to variable gain amplifiers.

[0068] Also, the gain distribution circuit 112 is a circuit having afunction of distributing the gain output data inputted from the gainconverting circuit 113 to a plurality of variable gain amplifiers asgain control data and controlling the data. In this mode for carryingout the invention, the circuit is constituted so that that the gainoutput data (Gain Output), either as it is or uniformlyamplified/attenuated, is distributed and supplied to a plurality ofvariable gain amplifiers.

[0069] Here, if the Gain Input has a large change, e.g., 48 dB ischanged by 24 dB to 72 dB and this change is reflected on the set valuesof the variable gain amplifiers, respectively, a high transient voltageis generated by a DC offset as already described in detail in “Problemsthat the Invention is to Solve” part.

[0070] Taking this into consideration, in this mode for carrying out theinvention, the upper limit or the maximum step of a gain (MAXSTEP) whichcan be changed once is set. For example, MAXSTEP is set at 2 dB. Bysetting so, the quantity of the change of a gain of 24 dB is attainedfrom 12 quantities of change each of 2 dB at intervals of pre-determinedperiods (Pre-Determined Periods). As a result, the generation of atransient voltage can be effectively suppressed.

[0071] For example, FIG. 8 shows the transient voltage if V_(of3) is 1mV and the gain g₃ of the VGA3 is changed from 1 times (0 dB) to 16times (24 dB) as high as the input. The transient voltage at a peak of 1mV×(16−1)=15 mV is generated.

[0072] If the gain g₃ is changed from 22 dB by 2 dB to 24 dB, a voltageX (dBm) at 1 mV and 22 dB can be obtained as 1 mV×10^((22/20)) from 20log₁₀X=22 dB. Therefore, to change the gain by 2 dB, it suffices thatthe voltage is 1 mV×(16−10^((22/20)))=3.4 mV.

[0073] The gain converting circuit 113 outputs such gain output data, asgain set value, to the gain distribution circuit 112 and the gaindistribution circuit 112 distributes the gain set value to therespective variable gain amplifiers 102, 103 and 104 either as it is orwhile amplifying/attenuating the value.

[0074] As stated above, the gains of the respective variable gainamplifiers are controlled a plurality of times at intervals ofpre-determined periods based on the upper limit of the maximum quantityof change or MAXSTEP, whereby the peak value of a transient voltage canbe greatly reduced.

[0075]FIG. 2 is a flow chart showing the operation of the gainconverting circuit 113 in the first mode for carrying out the invention.If the inputted Gain Input value is higher than the Gain Outputcurrently set to the variable gain amplifiers by MAXSTEP dB or more(‘YES’ in a step s1), then the MAXSTEP dB is added to the currently setGain Output to obtain a newly set Gain Output (in a step s4).Conversely, if the inputted Gain Input value is lower than the currentlyset Gain Output by MAXSTEP dB or more (‘NO’ in the step s1 and ‘YES’ inthe step s2), then MAXSTEP dB is subtracted from the currently set GainOutput to obtain a newly set Gain Output (in a step s6). In the othercase (‘NO’ in the step s1 and ‘NO’ in the step s2), the currently setGain Input becomes a newly set Gain Output (in a step s3). Thisoperation is carried out in each pre-determined period (Pre-DeterminedPeriod) (in steps s5 and s7) until the Gain Output becomes equal to theGain Input.

[0076] As a result, the quantity of the change of the Gain Output ineach pre-determined period is limited to MAXSTEP dB or lower.

[0077] It is also possible to determine the maximum number of times forcirculating the flow in the flow chart of FIG. 2 so as not to change theGain Output if the number of times exceed the maximum number.

[0078]FIG. 3 shows a manner in which the Gain Output follows the GainInput. In the example shown in FIG. 3, the Gain Input greatly increasescompared with the Gain Output at a time t=0 and, therefore, the GainOutput increases by MAXSTEP in each Pre-Determined Period. At a timet=t1, the Gain Input and the Gain Output satisfy Gain Input≦GainOutput+MAXSTEP. In this case, they also satisfy Gain Input≧GainOutput−MAXSTEP. Thus, the Gain Output is set to be Gain Input (GainOutput=Gain Input). Thereafter, the Gain Input decreases to lower level,so that after the time t=t1, the Gain Output decreases by MAXSTEP ineach Pre-Determined Period and at a time t=t2, the Gain Output is set tobe Gain Input (Gain Output=Gain Input).

[0079] As can be seen, if the limiting value is set for the quantity ofthe change of a gain which can be changed once and the change of a gainexceeding the limiting value is controlled, then control is carried outfor attaining a required gain change as a plurality of times of changesof the gain each equal to or lower than the limiting value of the gainchange. That is, a plurality of quantities of the change of the gain andthe quantity of the change of the last gain equal to or lower than thelimit value are controlled.

[0080] According to the gain control in this mode for carrying out theinvention, therefore, even if the Gain Input is greatly changed, thechange of a gain is gradually made for a long period of time and thechange of the Gain Output can be, therefore, decreased. Due to this,even if there is an offset in a plurality of variable gain amplifiers,it is possible to suppress the generation of a sudden transient voltage.

[0081] (Second Mode of the Invention)

[0082] In the mode for carrying out the invention stated above,description has been given to a case where the gain distribution circuit113 uniformly distributes gain control data to a plurality of variablegain amplifiers. It is also possible to greatly suppress the generationof a transient voltage by a distribution method by the gain distributioncircuit 113.

[0083] In the second mode for carrying out the invention, the gaindistribution circuit 112 exercises control so as to supply differentgain control data to a plurality of variable gain amplifiers.

[0084] Assume that the gain converting circuit 113 outputs gain data asgain control data as it is. The example shown in FIG. 10, for example,shows the transient voltage if V_(of1) is 1 mV, the gain g₁ of the VGA1is changed from 1 time (i.e., 0 dB) to 16 times (24 dB) as high as theinput and the gains g₃ and g₂ are the maximum gain of 24 dB,respectively. The transient voltage at a peak as high as

1 mV×(16−1)×16×16=3840 mV

[0085] is generated.

[0086] This is because the gains g₃ and g₂ are the maximum gain of 24dB, respectively. To prevent this, the gain distribution circuitdistributes a gain as follows.

[0087] For example, if the gain of a certain VGA_(X) is changed, thegains of all the VGA's provided right of the VGA_(X) are set at aminimum gain, respectively.

[0088] According to the example of the present invention, the minimumgain is 0 dB. By so controlling, a transient voltage is limited to

1 mV×(16−1)×1×1=15 mV.

[0089]FIG. 4 is a flow chart showing the operation of the gaindistribution circuit 112 in the second mode for carrying out theinvention. In the control conducted by the gain distribution circuit 112in this mode for carrying out the invention, a control algorithm thatthe gains of variable gain amplifiers starting at a variable gainamplifier close to an input are sequentially increased and that, if thegain is decreased, the gains of variable gain amplifiers starting at avariable gain amplifier farthest to the input are sequentially decreasedis utilized. The gain distribution circuit 112 exercises control so thatthe maximum value of the amplification gains of the variable gainamplifiers in this mode becomes, for example, 24 dB, threshold valuesare set at 24 dB and 48 dB for the input gain data (Gain) of the gaindistribution circuit and that the respective variable gain amplifiershave different gains according to the state of the input gain data.

[0090] In a step s11, it is judged whether or not the Gain inputted tothe gain distribution circuit 112 is higher than 48 dB. If the Gain ishigher than 48 dB, then the VGA1, VGA2 and VGA 3 are set at 24 dB, 24 dBand (Gain−48 dB), respectively and the operation returns to the steps11. If it is judged that the Gain is equal to or lower than 48 dB inthe step s11, it is then judged whether or not the Gain is higher than24 dB in a step s12. If the Gain is higher than 24 dB, then the VGA1,VGA2 and VGA3 are set at 24 dB, (Gain−24 dB) and 0 dB, respectively andthe operation returns to the processing of the step s11. If the Gain islower than 24 dB, then the VGA1, VGA2 and VGA3 are set at Gain, 0 dB and0 dB, respectively and the operation returns to the processing of thestep s11.

[0091] If gain is distributed to the VGA1, VGA2 and VGA3 as therespective gain control data of a plurality of variable gain controlamplifiers as shown in FIG. 4 and a certain VGA is changed, data iscontrolled so that the gains of the VGA's on the output side relative tothe certain VGA become a minimum gain, respectively.

[0092] In the examples of gain distribution to the VGA1, VGA2 and VGA3in this mode for carrying out the invention, the gain distributioncircuit exercises control based on the control algorithm that if thegain control data from the gain converting circuit 113 or Gain is high(higher than the threshold value of 48 dB), the gains of the VGA1 andthe VGA2 closer to an input are increased to 24 dB and the gain of theVGA3 is the remaining gain of (Gain−48 dB) to satisfy VGA1, VGA2>VGA3,that if the Gain is an intermediate (higher than the threshold value of24 dB and equal to or lower than the threshold value of 48 dB), theVGA1, VGA2 and VGA3 are set to satisfy VGA1 (=24 dB)>VGA2 (=Gain−24dB)>VGA3 (=0 dB) and that if the Gain is low (lower than 24 dB), theVGA1, VGA2 and VGA3 are set to satisfy VGA1 (=Gain)>VGA2 (=0 dB), VGA3(=0 dB). In short, if the gain is to be increased, the gains of variablegain amplifiers starting from the variable gain amplifier close to aninput are sequentially increased. If the gain is to be decreased, thoseof variable gain amplifiers starting from the variable gain amplifierfarthest to the input are sequentially decreased.

[0093] (Third Mode of the Invention)

[0094] As a mode for carrying out the invention for suppressing atransient voltage more effectively in the gain control of the presentinvention, the first mode and the second mode for carrying out theinvention are combined. A combination of gain control while setting anupper limit for the quantity of gain change and distribution control fordistributing different gains to a plurality of variable gain amplifiersaccording to the increase and decrease of the gain are conducted,thereby making it possible to realize the effect of suppressing atransient voltage in a multiplied manner.

[0095] In this mode for carrying out the invention, a gain convertingcircuit 113 is constituted to carry out an operation with a maximum stepprovided at the circuit 113 as in the case of the first mode forcarrying out the invention. A gain distribution circuit 112 isconstituted to distribute gain control data as in the case of the secondmode for carrying out the invention.

[0096] While a transient voltage is suppressed down to 1mV×(16−1)×1×1=15 mV in the second mode for carrying out the invention, atransient voltage can be further decreased to

1 mV×(16−10^((22/20)))×1×1=3.4 mV

[0097] in this mode for carrying out the invention.

[0098] While description has been given using flow charts so far, thefunction of the flow charts can be realized by a hardware by describingthe function as it is with a functional description language such asVHDL.

[0099] (Another Mode of the Invention)

[0100] As another mode for carrying out the present invention, a gaindistribution circuit 112 can be constituted out of an ROM while thebasic constitution of the circuit is the same as those described above.

[0101]FIG. 5 shows an example in which the gain distribution circuit isconstituted out of an ROM. As shown in FIG. 5, while a gain set valuefrom a gain converting circuit 113 is set as an address input, the gainsof respective variable gain amplifiers corresponding to the addressinput are read from the ROM and thereby set. Data written to the ROM ispre-determined so as to satisfy the algorithm of FIG. 4, whereby thegain distribution circuit can operate in the same manner as that in thepreceding modes for carrying out the invention.

What is claimed is:
 1. A gain control method for maintaining a totalamplifying gain of an output from a baseband amplifier includingseries-connected variable gain amplifiers (VGAs), by using a prescribedgain limit and a prescribed time period, which comprising the steps of:fixing a prescribed gain for said output from said baseband amplifyier;comparing said total amplifying gain with said prescribed gain; addingsaid prescribed gain limit to said prescribed gain when said totalamplifying gain is higher than said prescribed gain by said prescribedgain limit or more, or subtracting said prescribed gain limit from saidprescribed gain when said total amplifying gain is lower than saidprescribed gain by said prescribed gain limit or more; and repeating theadding step or the subtracting step until said total amplifying gainbecomes equal to said prescribed gain.
 2. The gain control methodaccording to claim 1, wherein the repeating step is limited to aprescribed number.
 3. The gain control method according to claim 1,wherein said prescribed gain is distributed in such a manner that thegains of said VGAs are increased in serial order from the input to theoutput of said baseband amplifier when said total amplifying gain is tobe increased, while the gains of the VGAs are decreased in serial orderfrom the output to the input of said baseband amplifier when said totalamplifying gain is to be decreased.
 4. A gain control method formaintaining a total amplifying gain of an output from a basebandamplifier including series-connected variable gain amplifiers (VGAs), byusing a prescribed gain limit and a prescribed time period, whichcomprising the steps of: fixing a prescribed gain for said output fromsaid baseband amplifyier; comparing said total amplifying gain with saidprescribed gain; distributing said prescribed gain in such a manner thatthe gains of said VGAs are increased in serial order from the input tothe output of said baseband amplifier when said total amplifying gain isto be increased, while the gains of the VGAs are decreased in serialorder from the output to the input of said baseband amplifier when saidtotal amplifying gain is to be decreased.
 5. A baseband amplifierincluding series-connected variable gain amplifiers (VGA) formaintaining a total amplifying gain, which comprises: a gain convertingcircuit for fixing a prescribed gain for the output from said basebandamplifyier; for comparing said total amplifying gain with saidprescribed gain; for adding a prescribed gain limit to said prescribedgain when said total amplifying gain is higher than said prescribed gainby said prescribed gain limit or more, or subtracting said prescribedgain limit from said prescribed gain when said total amplifying gain islower than said prescribed gain by said prescribed gain limit or more;and for repeating the adding step or the subtracting step until saidtotal amplifying gain becomes equal to said prescribed gain; and a gaindistribution circuit for distributing the output from said gainconverting circuit.
 6. The baseband amplifier according to claim 5,wherein said output from said gain converting circuit is equallydistributed to said VGAs.
 7. The baseband amplifier according to claim5, wherein the repeating step is limited to a prescribed number.
 8. Thebaseband amplifier according to claim 5, wherein said gain distributioncircuit distributes said prescribed gain in such a manner that the gainsof said VGAs are increased in serial order from the input to the outputof said baseband amplifier when said total amplifying gain is to beincreased, while the gains of the VGAs are decreased in serial orderfrom the output to the input of said baseband amplifier when said totalamplifying gain is to be decreased.
 9. A baseband amplifier includingseries-connected variable gain amplifiers (VGA) for maintaining a totalamplifying gain, which comprises: a gain converting circuit for fixing aprescribed gain for the output from said baseband amplifyier and forcomparing said total amplifying gain with said prescribed gain; and again distribution circuit for distributing said prescribed gain in sucha manner that the gains of said VGAs are increased in serial order fromthe input to the output of said baseband amplifier when said totalamplifying gain is to be increased, while the gains of the VGAs aredecreased in serial order from the output to the input of said basebandamplifier when said total amplifying gain is to be decreased.